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Screw Conveyor Systems

Design, application and selection guidelines.

The screw conveyor is one of the most reliable and cost-effective ways for conveying bulk materials. It is a very versatile machine that can
handle a wide variety of materials from dry, free-flowing materials such as portland
cement to wet, sluggish materials such as dewatered biosolids. The purpose of this article
is to help the reader understand the basics of screw conveyor design and proper
selection of the right screw conveyor for their applications.

History of the screw conveyor

Archimedes designed the first screw conveyor in the third century B.C. It was used for
removing water from ships and for irrigating farmland. The device consisted of a hollow
cylinder with a center shaft and a spiral fixed to the inner wall of the cylinder and
center shaft. As the assembly rotated, water was conveyed and lifted from one location
to another. The spiral design is based on the theory of the inclined plane.

The screw conveyor began to evolve in the late 1800s and was
used as a means of increasing feed and grain production to serve
the needs of the rapidly growing American population. The first
feed mills utilized screw conveyors throughout the process. Even
the most modern feed mills today depend on screw conveyors
for many of their material handling requirements. The screw conveyor
has evolved to modern times. It is now used in almost every
major industry. Today, thousands of processing plants throughout
the United States and the world are using screw conveyors to
convey bulk materials from A to Z (Adipic Acid to Zinc Concentrate).

Definition of Bulk Materials

Bulk materials are classified by particle size, flowability and
other physical properties. The Conveyor Equipment Manufacturers
Association (CEMA) provides material classification codes for
most commonly conveyed bulk materials. This information is
available in CEMA Book No. 350.

For example, the CEMA Book No. 350 indicates that the material
code for portland cement is 94A10026M. The CEMA Material
Classification Code Chart describes the material as weighing 94
lbs. per cubic foot, with a very fine particle size, that is free flowing,
moderately abrasive and can become aerated or fluid when
conveyed. The CEMA Material Classification Code Chart is to be
used as a general guideline for screw conveyor design.

Basic Theory and Design

Screw conveyors are volumetric conveying devices. With each
revolution of the screw, a fixed volume of material is discharged.
The purpose of a screw conveyor is to transfer product from one
point to the next. Screw conveyors are always control fed at the
inlet by another conveyor or metering device. Rotary valves,
screw feeders, belt conveyors, grinders, or even other screw conveyors
typically are connected to the inlet of a screw conveyor.
Screw feeders are similar to screw conveyors except that screw
feeders are always flood loaded or 100-percent full in the inlet
area. Screw feeders are designed to volumetrically meter material
from a hopper, bin or silo at a controlled rate. Many screw feeders
utilize adjustable speed drives to allow for varying the material
flowrate.

The flowrate or capacity of a screw conveyor is measured in
cubic feet per hour. If the capacity is given in lbs. per hour, tons
per hour or bushels per hour, it is converted to cubic feet per
hour. Since screw conveyors are control fed at the inlet, the crosssectional
trough loading is less than 100-percent. CEMA has
developed standards for trough loading based on the material

classification codes. Using the previous example of portland
cement, it is described as free flowing and moderately abrasive.
CEMA recommends a trough loading of no more than 30-percent.
CEMA also recommends reducing the speed of the screw conveyor
when conveying mildly to extremely abrasive materials.
Reducing the trough loading and speed will reduce the wear on
the screw conveyor. This information is readily available in the
CEMA No. 350 Book, from KWS Manufacturing Company or
another CEMA approved supplier. The percentage of trough
loading is based on the material being conveyed and if internal
hanger bearings are used. Hanger bearings are located inside the
conveyor and are used to support the screw.

There are certain parameters for sizing a screw conveyor. The
capacity calculation takes into account the outside diameter of
the screw, the outside diameter of the pipe, the pitch of the screw
and the trough loading. The calculation determines the capacity
in cubic feet per hour that will be conveyed with each revolution
per minute of screw rotation. It is not necessary to memorize this
calculation. Most CEMA approved screw conveyor manufacturers
have the capacity calculation and the CEMA guidelines as part of
their screw conveyor design software.

Choosing the right screw conveyor for your application
requires basic knowledge of the material being conveyed as discussed
previously, as well as, some basic information such as the
conveyor length, degree of incline and product temperature.

Applications and Industries

Screw conveyors are used in thousands of applications
throughout most industries. The major industries that utilize
screw conveyors are:

Food Processing

Pulp and Paper

Petroleum Refining

Stone, Glass and Concrete

Environmental

Wood Products

Chemicals

Rubber and Plastics

Primary Metals

Within each of these major industries are more specific applications
for screw conveyors. For example, the production of portland
cement is classified as part of the stone, glass and concrete industry.
The production of limestone is also found under the same industry.
Each major industry can have as many as 20 more specific industries
that use screw conveyors in their processes. Another example is the
use of screw conveyors in the meat and poultry processing industries.
These two industries are part of the major industry of food processing.
Screw conveyors are used throughout the meat and poultry
processing industries for conveying the byproducts or rendered
products.

The production of gypsum wallboard or sheetrock requires the
use of many screw conveyors. The gypsum wallboard industry is part
of the stone, glass and concrete industry. The image below is a typical screw
conveyor used for conveying stucco or gypsum. Construction features
include heavy-duty carbon steel construction, dust-tight covers,
adjustable shaft seals and pillow block bearings. The conveyor is
constructed from thicker materials due to the moderately abrasive
nature of gypsum. Since the gypsum is very dusty, the dust-tight
covers and adjustable shaft seals keep the material contained within
the screw conveyor. The pillow block bearings are mounted on
pedestals away from the conveyor to avoid contamination.

Recent Developments

As materials of construction
such as abrasion-resistant alloys and stainless steels have been
developed over the years, the use of screw conveyors has
increased to many more industries as previously discussed.
Recent production technology improvements include automating
specific manufacturing processes that improve quality and
reduce costs. The use of automated or robotic welding in the
screw-making process is a good example.

One of the most recent developments is the shaftless screw
conveyor which utilizes a screw without the center pipe. The
screw or spiral is constructed from high-strength steel and is
rigid enough to handle the drive torque. The spiral typically lies
in the bottom of the trough on a liner made from UHMW plastic
or metal. The drive shaft is connected directly to the shaftless
spiral. The shaftless conveyor was originally developed to
convey wet and sticky materials. When conveyed with a shafted
screw conveyor, wet and sticky materials have a tendency to
stick to the pipe where the flight is attached.
Since the center pipe is eliminated in a
shaftless conveyor, there is no place
for the material to stick. The image below
shows a typical shaftless
screw conveyor.

Differences Between Manufacturers

Each screw conveyor manufacturer is unique and provides
different products and services. Some manufacturers specialize
in providing CEMA standard stock components while others
focus on custom designs. It is important to choose the proper
screw conveyor manufacturer based on your specific needs and
the customers in your area. For example, if you have several rendering
plants in your area, you will need a manufacturer that
specializes in designing and building heavy-duty screw conveyors.
Material thickness for screw flights and troughs can be as
thick as one inch. Manufacturers that regularly build screw conveyors
for the rendering industry must have special flight presses
and other equipment for forming and handling one-inch
thick plate. Also make sure the manufacturer that you choose
has a written quality control program and is ISO 9001-2008 certified
so you know you are getting quality equipment. Welders
must also be certified to either AWS or ASME code standards.

The major screw conveyor manufacturers are active members of
CEMA and can be located by going to www.cemanet.org. You
can contact CEMA for information if you are not familiar with a
certain manufacturer. Also, check out the individual company's
website or contact the company directly to discuss capabilities
and quality programs.

Adding Value for the Customer

Screw conveyors are an important part of your customer's
operations. If a screw conveyor continually fails or requires
excess maintenance, then your customer is not efficiently producing
product. Having a basic knowledge of screw conveyor
design along with understanding the application will help you
in solving the customer's problem. For example, your customer
is conveying petroleum coke and replacing screws every three
months. The CEMA material classification code for petroleum
coke is 30D737N. The code indicates that the petroleum coke
weighs 30 lbs. per cubic foot, has a large lump size, has average
flowability, is extremely abrasive and can be explosive. The
screws are wearing out prematurely because petroleum coke is
extremely abrasive. As an added value, you can offer screws
constructed from abrasion-resistant materials or the screw
flights can be hardsurfaced with a weld-on hardsurfacing
material.

The right image shows a screw conveyor with weld-on hardsurfacing
on the screw and internal surfaces of the housing.
IDC University offers a two-day course in screw conveyor
design and applications. Applications such as those mentioned in
this article will be discussed in detail and you will get hands-on
experience with screw conveyors. You will gain important knowledge
about various industries and applications that will help you
increase sales and make your company more profitable.

Bill Mecke, P.E. is Vice-President and owner of KWS Manufacturing
Company, Ltd. He has over 20 years of experience in the bulk
material handling industry and in the design of screw conveyors.
Bill is currently the chairman of the CEMA Engineering Committee
for Screw Conveyors. He received his Bachelor of Science degree in
Mechanical Engineering from Texas A & M University and
is a professional engineer in the state of Texas.